[lint request] unnecessary_str_transmute

Some people transmute a (u8* [const, mut], usize) into a &str/&mut str/* const str/*mut str unnecessarily and vice-versa, instead of building a slice first (e.g. using the from_raw_parts methods), and then using the from_utf8 or from_utf8_unchecked str methods (yeah, really).

We should probably cover also the case where the pointer type is not an u8 since the transmutes will work for any pointer type. People should use the proper methods instead of relying on the unspecified str type layout.

We should detect this and tell people to use the proper methods instead. We probably want to detect the incorrect (usize, T* [const, mut]) cases as well, and tell people to also use the correct methods instead.

If people are doing this inside a macro... we should warn anyways.

The issue provides no file or test entry point, so start by tracing how Rust Clippy defines and tests existing transmute lints. Done means detecting the listed pointer/string transmute forms, including non-u8 pointers and reversed tuple order, and warning even when the code appears inside a macro.